The present invention relates to apparatus for measuring the distance from a metallic body to be measured by utilizing the effect of the eddy currents induced in the metallic body, and more particularly the invention relates to improvements in such apparatus which ensure an increased measuring span.
An apparatus has been disclosed in U.S. Pat. No. 4,030,027 in which a reference signal having a fixed frequency and fixed amplitude is applied to the inverting input terminal of a differential amplifier whose output is applied to its noninverting input terminal through a positive feedback circuit and the feedback signal is also applied to a detection coil, whereby the distance from a metallic body disposed opposite to the coil is measured in accordance with a change in the coil impedance. Where the detection coil of this apparatus consists of a cylindrical coil, while the measuring span increases with an increase in the inner diameter of the coil, it is impossible to increase the coil inner diameter if there are limitations on the size or dimension of objects to be measured, the space available for the measuring means and the like. On the other hand, where a plurality of interchangeable coils are used with the measuring apparatus, it is advantageous from the design standpoint of the coil mounting structure to make the size of all the coils equal to one another as far as possible.